What is the X in X-mas?
1660s, from Medieval Latin vesicantem (nominative vesicans), present participle of vesicare, from vesica "a bladder, a blister" (see ventral). From 1826 as an adjective.
vesicant ves·i·cant (věs'ĭ-kənt)n. A blistering agent, especially mustard gas, used in chemical warfare. adj. Causing blisters.